This invention relates to filter press apparatus, and more particularly to such apparatus comprising a plurality of substantially parallel filter plates assembled as a unit or pack, and housed within a substantially rigid outer shell to constitute a closed pressure vessel, either or both of the unit and the shell being movable relative to the other between a position wherein the unit is sealed within the shell to enable pressurised supply to the shell of medium to be filtered via the plates, and a position wherein the unit is at least partially removed from the shell to enable the discharge of resultant filter cake from between the plates.
UK patent no. 2282977 discloses apparatus of this type and incorporating a cylindrical flexible membrane within the shell which is inflated to urge the membrane against the periphery of the unit to define a substantially incompressible boundary wall thereto for the duration of the filtration operation. When the filtration operation is completed, and it is desired to remove the unit from the shell to enable discharge of the filter cake, the membrane is deflated whereby a gap is formed between the shell/membrane and the unit. Thus the unit can be removed from the shell without interference between the shell and the unit, and the filter cake can be removed from the unit.
After completion of the filtration operation, and prior to removal of the unit from the shell to enable discharge of the filter cake, it is often desirable to be able to compress the unit to squeeze excess moisture from the filter cake.
This is achieved by means of hydraulic rams, inflatable bags or the like which compress the unit axially while the membrane is in contact with the unit.
Such an arrangement works well with units having a relatively small number of filter plates or when used with filter cakes that are not very compressible, in which cases there is little stress on the membrane due to the limited axial movement of the unit.
However, in cases where the cakes are relatively compressible and/or many filter plates are present, the first filter plate of the unit moves a relatively large distance during the compression stage, and, as the membrane is urged against the unit, undue stress can be applied to the material of the membrane which can cause rupture of the membrane.